1. Field of the Invention
The present invention relates generally to methods of heat treating such as induction tempering and particularly to a method of induction tempering that performs the tempering at the same location as hardening in the heat treating process, termed "in-situ" tempering.
2. Discussion
Brazing is a well-known method of joining two metal pieces by fusing a layer of alloy between adjoining surfaces. It also is known to heat the brazing alloy layer in the region of the joint by use of an induction coil encircling the joined members in the plane of the brazing alloy layer (flux) for its fusion. The energy supplied to the induction coil typically is controlled by either visual operation of the heating effect on the brazing ring or by time, that is, the coil is simply powered for a predetermined time, with hopefully the desired result achieved.
This approach leads to problems, particularly when brazing is done of thin-walled members such as condenser tubes of the type used in automotive applications. Such tubes typically are brazed to the condenser via sockets and are constructed of materials with complex chemistry such as aluminum. In such cases, brazing typically is performed with an aluminum alloy. This combination of a thin-walled tube and the complex chemistry of aluminum makes for a difficult mix. For example, it is extremely difficult to control the temperature of aluminum and it is easy to melt through a thin-walled tube during the brazing process. Under heating causes inadequate penetration of the brazing ring into the brazed joint. Such heating variations are exacerbated by variations in the tubing wall dimension, variations in the amount of flux applied, variations in the air gap between the tube and the socket, and variations in the tube's position relative to the induction coil. High scrap rates have resulted from the problems inherent in such brazing processes.
For example, aluminum condensers have been induction brazed by Chrysler Corporation in a manufacturing process typically consisting of brazing 1/2 inch or 5/8 inch header tubes to a condenser socket. The tubes are manually assembled with a braze ring of an aluminum alloy prior to presentation at the induction machine. Up to 40% of the product typically either requires rework or is scrapped due to heating variations of the brazed joint.
Thus it would be desirable to provide a method and apparatus of induction brazing wherein the energy supplied to the braze alloy could be controlled in a manner to minimize defective brazed joints.
It would further be desirable to provide a method and apparatus for induction brazing wherein the power supplied to the induction coil could be controlled by a signal that is indicative of the state of the brazed joints.
Accordingly, the invention disclosed in the parent application provides a method and apparatus for induction brazing in which the power supplied to the induction coil is controlled by a closed-loop temperature controller that is controlled by the temperature in the vicinity of the brazed joint. The use of a closed-loop temperature controller allows for a uniform heat input to the brazed joint, eliminating production scrap or rework. By using the method and apparatus of the present invention Chrysler has increased its throughput from 60% to 90%. In the present invention an optical pyrometer senses the emitted radiation from the brazed joint. A controller conditions the input and provides a signal to the power supply for output power control. The controller is programmed to control around a specific set point which corresponds to the desired brazing temperature of the base alloy. With closed loop control the brazing process is rendered insensitive to process variation. Additionally, flux drying ovens no longer are required in the brazing process.
The method and apparatus of the invention disclosed in the parent application comprises brazing a first member to a second member via an inductive coil placed around a brazing ring of alloy at the brazed joint connection of the two members. Temperature at the brazed joint is sensed via an optical pyrometer and the power to the induction coil is controlled via a closed loop controller to bring the temperature to a specific set point corresponding to the desired brazing temperature of the alloy.
It is further a common practice to increase the strength of carbon steel by heat treatment of the steel. The invention disclosed in this Continuation-in-Part application relates more specifically to this practice, and uses technology related to the method and apparatus disclosed in the parent application. One method of heat treating carbon steel changes the steel's microstructure to produce martensite, which is an extremely hard and brittle phase of the steel that forms when the steel's temperature is raised above a certain critical temperature, for example, 1550.degree. F. for 1050 steel, and then rapidly cooled to room temperature. The resulting martensitic steel is strong but relatively brittle due to internal residual stresses resulting from the uneven, rapid cooling and martensite expansion. Tempering of the steel is an additional known step in the hardening process that is done following the formation of the martensite and that relieves the residual stresses and results in a tougher, slightly weaker microstructure called tempered martensite.
Tempering typically is done at lower temperatures than the initial heating (300.degree. F.-700.degree. F.) for periods of up to one hour. Tempering typically is accomplished through the use of a large furnace, which provides a uniform distribution of heat throughout the tempered components. The large furnace that typically is used requires a large amount of space. Furnace tempering further involves a multistep procedure wherein the parts are first heat treated to form martensite in one location, removed from that location, moving to the tempering furnace, and then tempered. This multistep process obviously involves extensive manpower and equipment resources.
Known induction tempering techniques also control the tempering only by controlling the time the part is tempered, often resulting in overtempering and subsequent weakening of the part.
Accordingly, it is a principal objective of the present invention to provide a method and apparatus for tempering in which the tempering is performed at the same location that the initial heat treating occurs, through use of an induction coil, termed "in-situ" tempering. An induction coil and apparatus similar to that used ill the brazing operation disclosed in the parent application is used to perform the in-situ tempering of the present invention.
It is another object of the present invention to provide a method and apparatus for in-situ tempering that uses closed loop temperature control to prevent overtempering and that allows uniform tempering to be obtained throughout the case depth of a part and not limited to simply a "surface temper" which is typically found in industry.